Vinylidene fluoride homopolymers are thermoplastic resins noted for their mechanical, thermal, electrical and chemical properties. They differ from other prior art fluoro polymers by having a greater capacity for production using conventional techniques for transformation of thermoplastics.
However, such capacity for production is relative. Actually, only polyvinylidene fluorides of slight or medium molecular weight are moldable or extrudable. On the other hand, those with high or very high molecular weights, because of high viscosity in the molten state do not have capacity for production by conventional techniques.
On the other hand, production becomes difficult, if not impossible when fillers are to be incorporated into the structure of the polyvinylidene fluorides. Sometimes it is necessary to add filler either to cut cost or improve particular properties such as dimensional stability and thermal resistance. Depending on the nature, amount or form of the fillers added, the rheological characteristics of the resulting mixtures are modified and most often make production difficult.
Prior art fillers, that tend to improve the rheological characteristics of vinylidene fluoride homopolymers, have been described in French patent Nos. 2,069030 and 2,019,492. French patent No. '030 discloses the use of low or high density polyethylene in a homogeneous mixture with polyvinylidene fluorides. These types of polyolefin actually contribute a notable lubricating effect but have very limited compatibility with a base resin, reflected by the opaque tint of its mixtures, and its slight thermal stability, leading to a rapid degradation, limit use of these polyolefin types. French patent No. '492 teaches the incorporation of high molecular weight polytetrafluoroethylene in the form of fine particles. However, the presence of this latter polymer, while making possible the elimination of extrusion defects known as "melt fracture" and "die fracture", causes an increase in the viscosity of the polyvinylidene fluoride in the molten state. This increased viscosity, corresponding to a reduction in fluidity at high temperatures, is particularly undesirable in most of the conventional processing techniques.
The essential principles of injection of expanded thermoplastics are described by Clarence W. Wallace in Plastics Engineering of November, 1976. In particular, the author stresses the necessity of a high-speed injection to obtain a maximum expansion and a good surface appearance. However, high-speed injection produces undesirable heating at the passage in the injection nozzle and leads to thermal degradation of the polymer. Thus, expanded formulations require lubrication to avoid this phenomenon.
The works of H. Ebneth et al in Kunstoff Rundschau, No. 5 (May, 1971), show the importance of fluidity in the molten state during filling of a mold with expanded thermoplastic. The ratio between the maximum travel of the material, from the point of injection to the part most distance from this point and the thickness of the product are essential characteristics linked to ease of molding. The higher this ratio the more fluid the material should be. Expanded formulations should therefore be as fluid as possible to permit filling of large molds.